1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to an organic electroluminescent display (OELD) device and a method of fabricating an OELD device.
2. Discussion of the Related Art
Until recently, many display devices have employed cathode-ray tubes (CRTs) to display images. However, various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and electro-luminescent display (ELD) devices, are currently being developed as substitutes for the CRTs. Among these various types of flat panel displays, OELD devices are a luminescent display with such advantages as fast response time, high brightness and wide viewing angles.
FIG. 1 is a circuit diagram illustrating an OELD device according to the related art.
Referring to FIG. 1, an OELD device includes a switching thin film transistor STr, a driving thin film transistor DTr, a storage capacitor StgC and an organic electroluminescent diode E in a pixel region P.
A gate line GL extends along a first direction, a data line DL extends along a second direction crossing the first direction, and a power line PL is spaced apart from the data line DL. The gate line GL and the data line DL define the pixel region P.
The switching thin film transistor STr is disposed at a crossing portion of the gate and data lines GL and DL. The driving thin film transistor DTr is connected to the switching thin film transistor STr. The organic electroluminescent diode E is connected to the driving thin film transistor DTr. A first electrode of the organic electroluminescent diode E is connected to a drain electrode of the driving thin film transistor DTr, and a second electrode of the organic electroluminescent diode E is connected to a ground terminal. The power line PL is connected to a source electrode of the driving thin film transistor DTr. The storage capacitor StgC is connected to a gate electrode and the drain electrode of the driving thin film transistor DTr.
When a gate voltage having an on level is applied to the gate line GL, the switching thin film transistor STr is turned on. A data voltage is applied to the driving thin film transistor DTr though the data line DL and the switching thin film transistor STr so that the driving thin film transistor DTr is turned on. Accordingly, a diode current flows on the organic electroluminescent diode E. Even when the gate voltage has an off level, the storage capacitor StgC stores the gate voltage of the driving thin film transistor DTr. Accordingly, the diode current is maintained until the next frame.
OELD devices may be categorized as a top emission type and a bottom emission type according to the direction of light emission.
FIG. 2 is a cross-section view illustrating a bottom emission type OELD device according to the related art.
Referring to FIG. 2, the OELD device includes first and second substrates 10 and 30 facing each other. A seal pattern 40 is disposed at a periphery portion of the two substrates 10 and 30, and the seal pattern 40 attaches the two substrates 10 and 30 together.
A driving thin film transistor DTr is disposed on the first substrate 10. A first electrode 12 is connected to the driving thin film transistor DTr. An organic emitting layer 14 is disposed on the first electrode 12. The organic emitting layer 14 includes red (R), green (G) and blue (B) emitting layers 14a, 14b and 14c in respective pixel regions. A second electrode 16 is disposed on the organic emitting layer 14. A desiccant 32 is disposed on the second substrate 30 to absorb moisture.
FIG. 3 is a cross-sectional view enlarging a pixel region of FIG. 2. In FIG. 3, a second substrate of FIG. 2 is omitted for brevity of explanation.
Referring to FIG. 3, a semiconductor layer 55 is disposed on a substrate 50. The semiconductor layer 55 includes an active portion 55a and two ohmic contact portions 55b. A gate insulating layer 58 is disposed on the semiconductor layer 55. A gate electrode 63 is disposed on the gate insulating layer 58 corresponding to the active portion 55a. An interlayer insulating film 67 is disposed on the gate electrode 63. The gate insulating layer 58 and the interlayer insulating film 67 have first and second semiconductor contact holes 69 and 71 exposing the both ohmic contact portions 55b. Source and drain electrodes 75 and 77 contact the both ohmic contact portions 55b through the first and second semiconductor contact holes 69 and 71, respectively. A passivation layer 85 is disposed on the source and drain electrodes 69 and 71. A power line 79 is disposed on the interlayer insulating film 67.
An organic electroluminescent diode E is connected to the drain electrode 77. The organic electroluminescent diode E includes first and second electrodes 82 and 89 and an organic emitting layer 87. The organic emitting layer 87 emits light. The emitted light travels toward the substrate 50.
In the related art bottom emission type OELD device, the active layers of the switching thin film transistor and the driving thin film transistor are exposed to an exterior light, such as sunlight and light from various lamps, through the transparent substrate 50. Such an exposure negatively affects the characteristics of the switching and driving thin film transistors, such as leakage current, threshold voltage and mobility. Accordingly, the OELD device according to the related art may suffer from low contrast ratio, high power consumption and noise and the display quality may thus become degraded. In particular, the leakage current may go up to several nano amperes (nA), thereby delaying light emission and degrading the thin film transistors and organic electroluminescent diode.